1. Field of the Invention
The present invention refers to a device for large-area implantation of ions for an ion implantation unit.
2. Description of the Related Art
Ion sources are technically used in fields in which a large-area homogenous ion beam is required. This is the case e.g. in the fields of large-area ion implantation for doping semiconductors, tribology for surface hardening and ion beam-assisted coating. A device according to the generic clause is known from JP-A-2-20018.
In the field of ion implantation for highly integrated microelectronics there is a trend towards smaller ion energies in combination with larger implantation surfaces. The maximum integration with more than 10.sup.7 components per circuit requires a reduction of the pn junction depth. In CMOS technology, for example, an extremely large number of pn junctions having a depth of less than 100 nm is required for the drain and the source. The production of increasingly flatter pn junctions entails great technological difficulties especially in the case of an implantation of boron for p.sup.+ regions. The requirements which have to be fulfilled by ion implantation in this connection, e.g. a low ion energy, distinctly below 30 keV, a high ion current and a high throughput, a parallel beam with large implantation areas and a high homogeneity, are not fulfilled by the commercial units which are available at present.
A further performance characteristic for future semiconductor manufacturing devices is the integrability in so-called multi-compartment process units, which are also referred to as cluster systems. This necessitates the development of small process modules which can be supplied with process wafers by a central roboter. Such systems are advantageous insofar as a plurality of process steps can be carried out in one unit. Due to the increasing number of process steps carried out in semiconductor manufacturing processes and due to the higher requirements that are to be met as far as the flexibility of the production process is concerned, such units become more and more important. Conventional implantation units are not suitable for integration in a multi-compartment process unit because their dimensions are too large. The commercial units, which become more and more complex and expensive, also require a great deal of maintenance work and a large clean-room area, and this, in turn, results in high operating expenses.
In order to fulfil the above-mentioned requirements, a new ion beam technology is necessary. One possibility is offered by the so-called "plasma immersion ion implantation", abbreviated to PIII. In such units a plasma is formed in the implantation chamber by means of high-frequency electromagnetic excitations. The ion density in this area is normally 10.sup.10 to 10.sup.11 cm.sup.-3. By applying a negative voltage, which can amount to some kilovolts, the positive ions are accelerated from the plasma in the direction of a specimen to be processed. A PIII unit has a plurality of advantages, such as a high ion current, low processing costs, and it is a, compact system which requires little maintenance work and which is very suitable for use in multi-compartment process units.
Although the PIII process has already been known for a long time, it has not been able to gain acceptance in semiconductor-technology implantation processes up to now. The reasons for this are the diffuse energy distribution (energy contamination), the inhomogenous dose distribution and the contamination of the specimens with heavy metals or undesired kinds of ions.
Known ion sources for the generation of large-area ion beams are the high-frequency and the electron-cyclotron-resonance ion sources.
When high-frequency ion sources (RF ion sources) are used, the operating pressure is approx. 10.sup.-3 to 10.sup.-2 mbar. These ion sources are advantageous as far as their simple structural design and the small amount of power consumed are concerned. The high frequency is coupled in capacitively or inductively. If the plasma required has to be very clean, the plasma reactor used will consist of a quartz dome. The use of a quartz dome makes it more difficult to couple in the high frequency and to introduce the gas into the plasma chamber in a uniformly distributed manner. In order to avoid these disadvantages, which result in inhomogeneous ion distributions and higher operating pressures, many manufacturers use metal components in the plasma chamber. This use of metal components results, in turn, in metal contamination on the process wafers and excludes the use of this type of ion source for large-area ion implantation. The conventional large-area RF ion sources are therefore only suitable for ion etching. Since, in the case of etching, the contaminations occur on the surface, but are not implanted in the semiconductor, they can be removed.
Electron-cyclotron-resonance ion sources (ECR ion sources) show, due to the electron-cyclotron resonance, a higher plasma density (10.sup.12 to 10.sup.13 cm.sup.-3) than high-frequency plasmas (10.sup.10 to 10.sup.11 cm.sup.-3). Hence, these sources work at a lower process pressure in the range of 10.sup.-5 to 10.sup.-2 mbar. One disadvantage of ECR ion sources is the divergence of the ion beam caused by the magnetic field. Like in the case of RF sources, a quartz dome is required for reducing the contamination when ECR ion sources are used in the field of semiconductors. This has again the effect that it is more difficult to introduce the gas in a uniformly distributed manner and to couple in the microwaves. Hence, some manufacturers use metal components in the plasma chamber. The ECR sources are used for conventional ion implantation (with mass separation) and for etching processes. Up to now, it has not been possible to use them for large-area ion implantation in view of the hitherto unsolved contamination problems and in view of the high magnetic fields. In addition, the homogeneity of the plasma decreases as the size increases, since the ECR condition is only fulfilled at the boundary of the plasma in most cases. Such an ECR ion source is known e.g. from DE 3708716 A1.
A further known ion source is the so-called Kaufmann ion source, which is, however, unsuitable for many cases of use for reasons of contamination. A special problem arising when Kaufmann ion sources are used is the corrosion and the sputtering of the thermionic cathode.
In the prior art, ion beam units are known, which use so-called ion optics. Hitherto known ion optics for large-area ion beams are, however, limited to the extraction and the acceleration of ions from the plasma. These known ion optics cannot be used for deflecting a large-area ion beam without impairing the homogeneity of the ion beam and without introducing an additional contamination source into the ion beam. Since the hitherto known grids or deflection means consist of metal or graphite, the ion beam sputtering causes material to be removed, which then contaminates the semiconductor specimens. A further problem arising in connection with the hitherto used grids is that an image of the grid structure is formed on the process wafer. Extraction grids are described e.g. in DE 4315348 A1 and in DE 3601632 A1.
In the prior art implantation units are known in which an ion source is provided, which produces an ion beam that is thin in comparison with the specimen. These known implantation units comprise, in addition to the ion source, a mass separation, an acceleration tube, a deflection or raster unit and an implantation chamber. The mass separation units of these known ion implantation units consist of a so-called analysis magnet with a vacuum gap, delimited by the two poles, between which the thin ion beam emitted by the ion source passes; due to the influence of the magnets, an analysis of the ion beam takes place in such a way that specific ions within the ion beam are deflected more strongly from their trajectory due to the effect produced by the magnets so that these ions are prevented from passing through a separation slot which is arranged subsequent to the analysis region. Hence, the thin ion beam only contains the desired ions. Such ion implantation units are described e.g. in EP 0405855 A2 and in U.S. Pat. No. 5,396,076.
The disadvantage of these known implantation units is that they are not suitable for effecting large-area ion implantation so that, in these known implantation units, a complicated rastering of the specimens cannot be dispensed with. Furthermore, a large-area ion beam cannot be produced by these known implantation units and, consequently, these implantation units do not offer the possibility of deflecting such a large-area ion beam or of effecting a mass separation of such a large-area ion beam. A further disadvantage of this known ion implantation units is that they have a complicated structural design in the case of which two magnetic poles have to be provided in opposed relationship with one another for influencing the ion beam that passes between these poles. In addition, a slot structure, which allows the desired ion beam to pass, has to be provided at the output of the separation unit. The separation magnet of production units has a weight of several tons, and, consequently, it does not permit a compact and modern concept of the unit in question, e.g. multi-compartment process units.